1) Field of the Invention
The present invention relates to an objective lens system and, in particular, a Petzval objective, capable of simultaneous dual band imagery in both the 3-5 and 8-12 micrometer spectral ranges, for use with a dual spectral band detector, such as a quantum well detector.
2) Discussion of Related Art
Recent advancements in quantum well detector technologies permit the simultaneous processing and display of dual infrared band imagery with a single detector at a single focal plane. There are advantages of imaging each spectral band and the possibility of focusing both bands simultaneously may yield greater information for surveillance and/or target discrimination by imaging devices.
Imaging devices, search and track sensors such as forward looking infrared systems, and other types of optical devices which operate in the 3-12 micrometer spectral region use either reflective or refractive objective lens systems. An advantage of reflective optics is that they operate within a wide spectral band. However, reflective optics systems have several undesirable features, such as, for example, a more limited field of view and a large physical size. In addition, central obstructions in reflective optics limit the modulation transfer function (MTF), or sine wave response.
Additionally, efficient cold shielding is important with any cooled detector array. Reflective optics are not appropriate for wide field of view applications, particularly when efficient cold shielding is necessary.
In the past, refractive lens systems have been designed for limited ranges, e.g., either the 3-5 or the 8-12 micrometer bands. This is because the refractive indices of the component lens materials vary significantly between the two spectral bands. For example, germanium behaves like a crown (a low dispersion material) in the 8-12 micrometer wavelengths and like a ffint (a high dispersion material) in the 3-5 micrometer region. Systems designed to operate in one band exhibit intolerable chromatic aberrations in the other band. The design for either long wave (8-12 micrometers) or short wave (3-5 micrometers) lenses are commonplace, but the need for simultaneous dual band operation is a relatively new requirement.
The number of optical materials that can transmit wavelengths in the spectral range of interest for this application is limited, and many infrared lens materials are soft or water soluble, which make them unsuitable for tactical systems.
Several studies have been carried out to isolate optical material combinations that yield refractive lens designs that are color corrected over the entire 3-12 micron spectral region. The following publications describe such studies: Thomas H. Jamieson, “Ultrawide Waveband Optics”, Optical Engineering, Vol. 23, No. 2, April 1984, pages 111-116; M. O. Lidwell, “Achromatism of Lenses for Thermal IR”, Proceedings SPIE, Vol. 518, 1984, pages 73-80; and, M. Roberts and P. Rogers, “Wide Waveband Infrared Optics”, Proceedings SPIE, Vol. 1013, 1988, pages 84-91.
These publications describe optical design studies that employ many different infrared lens material combinations. The solutions described in these studies yield excellent color correction over the spectral range of 3-12 microns, but the material combinations are poor choices from the standpoint of durability, manufacturability and reliability.